1. Field of the Invention
The present invention concerns in general a hydraulic shock-absorber for vehicle suspension, particularly for motor vehicle suspension.
More specifically, the present invention concerns a so-called regenerative hydraulic shock-absorber, i.e. a hydraulic shock-absorber comprising a cylinder, a piston arranged slidably inside the cylinder so as to separate the space inside the cylinder into two variable-volume working chambers, and electric energy generating device for generating electric energy using the flow of a working fluid flowing into, or out of, the working chambers of the cylinder as a result of the movement of the piston in the cylinder. A hydraulic shock-absorber of this type makes it possible, therefore, to generate electric energy using the extension/compression movements of the shock-absorber, i.e. the relative movements between the wheel and the body of the vehicle, due for example to the irregularities of the road surface, to the oscillations of the vehicle body resulting from longitudinal and/or lateral acceleration, etc., besides performing the primary function of a vehicle shock-absorber, i.e. to damp the relative movements between the wheel and the body of the vehicle to guarantee the comfort of the vehicle's occupants and ensure good road-holding.
2. Description of the Related Art
In normal hydraulic shock-absorbers for vehicle suspension, i.e. in hydraulic shock-absorbers of non-regenerative type for vehicle suspension, the kinetic energy of the suspension is dissipated in the form of heat. The idea on which regenerative shock-absorbers are based is to exploit the energy which would otherwise be dissipated in the form of heat, to produce electric energy, to be used, for example, to supply devices and systems on board of the vehicle, to charge the vehicle's battery and to actively control the damping characteristics of the shock-absorber itself, ensuring at the same time the primary function of the shock-absorber.
A regenerative hydraulic shock-absorber is known, for example, from WO2009/129363 and EP1878598 and comprises a cylinder containing a hydraulic working fluid, a piston reciprocally movable inside the cylinder so as to split the cylinder into two variable-volume working chambers, a hydraulic motor, an electric generator connected to the shaft of the hydraulic motor for generating electric energy as a result of the rotation of this shaft, and a hydraulic circuit connecting the hydraulic motor to the two working chambers of the cylinder in such a way that as a result of the movement of the piston in the cylinder the working fluid flows through the hydraulic motor causing rotation of the shaft of the hydraulic motor, and thus driving the electric generator, and then returns to the cylinder.
Regenerative shock-absorbers of the type indicated above have first of all the disadvantage that their total efficiency in the conversion from kinetic energy to electric energy is significantly less than 100%, for example of the order of 30%, as a result especially of hydraulic and mechanical losses in the hydraulic motor. Furthermore, providing for each shock-absorber a hydraulic motor and an electric generator coupled thereto naturally makes the vehicle suspension system more complex, heavier and more expensive. In addition, it may be difficult to find the space required to arrange the hydraulic motor and the electric generator associated to each shock-absorber.
A regenerative hydraulic shock-absorber of the type known in the related art is described in DE 10 2008 030 577.
According to this known solution, the shock-absorber comprises a cylinder containing a hydraulic working fluid, a piston arranged slidably in the cylinder so as to split the cylinder into a first and second variable-volume working chamber, an auxiliary conduit in fluid communication on one side with the first working chamber and on the other with the second working chamber, a plurality of permanent magnets arranged slidably in the auxiliary conduit to reciprocally move along the auxiliary conduit, dragged by the working fluid flowing between the first and the second working chamber through the auxiliary conduit as a result of the reciprocating movement of the piston in the cylinder, and at least one electrical winding wound around a portion of the auxiliary conduit, in such a way that the movement of the permanent magnets along the auxiliary conduit causes the magnetic flux concatenated with the winding to change, thus generating an induced electromotive force in the winding. The auxiliary conduit has a flow cross-section whose area is less than the internal working cross-section area of the cylinder, i.e. the difference between the internal cross-section area of the cylinder and the cross-section area of the shock-absorber rod. The auxiliary conduit has an intermediate portion which extends parallel to the axis of the cylinder and has a length substantially equal to the stroke of the piston in the cylinder. Alternatively, the intermediate portion of the auxiliary conduit is inclined with respect to the axis of the cylinder.
This known solution has a series of disadvantages which make it practically unusable.
Firstly, given that the fluid flows in the auxiliary conduit with a speed equal to the speed of the piston multiplied by the ratio of the internal working cross-section area of the cylinder to the flow cross-section area of the auxiliary conduit (a ratio which in the known solution is considerably greater than one), and therefore the stroke of the magnets in the auxiliary conduit is equal to the stroke of the piston multiplied by this ratio, the magnets reach the end of their stroke in the auxiliary conduit (for example the magnet which slides along the intermediate portion of the auxiliary conduit comes to a stop against the upper end or the lower end of the conduit) well before the piston has reached the end-of-stroke position.
Secondly, since only a small number of turns of the electric windings are affected by the magnetic flux generated by the permanent magnets, the electromotive force generated by the shock-absorber is correspondingly small. Furthermore, the great length of the electric winding, equal to the length of the portion of auxiliary conduit around which the winding is arranged, entails a reduction in the electrical efficiency of the shock-absorber.